Filter assembly

ABSTRACT

A filter assembly for removing material that is entrained in a gas stream. The filter assembly comprises a filter element comprising a wall of a filtration medium which defines a hollow space, for a gas stream to flow from the space through the wall to be filtered. The wall includes a filtration layer, and a drainage layer located outside the filtration layer in which liquid separated from the gas stream can collect. The filter element comprises first and second end caps at opposite ends of the wall, the first end cap including an inlet for a gas stream to be supplied to the space within the wall. The filter assembly further comprises a housing in which the filter element can be received when in use. The housing has an inlet port for a gas stream to flow into the housing which communicates with the inlet in the first end cap, and an outlet port through which gas which has passed through the filtration medium can leave the housing. The drainage layer extends over at least a part of the external surface of the second end cap between the second end cap and the housing and the part of the drainage layer between the second end cap and the housing is locally compressed by at least one longitudinally extending fin.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is continuation of copending International ApplicationNo. PCT/GB05/002918 filed Jul. 26, 2005, which designated the UnitedStates, the disclosure of which is incorporated herein by reference, andwhich claims priority to Great Britain Patent Application No. 0417459.5,filed. Aug. 5, 2004.

BACKGROUND OF THE INVENTION

This invention relates to a filter assembly for removing material thatis entrained in a gas stream.

SUMMARY OF THE INVENTION

Filtration of gas in a compressed gas system is generally required sothat the gas is sufficiently clean for a subsequent application or tominimise adverse effects of impurities on components of the system. Forexample, removal of compressor oil can be required to minimise chemicalcontamination and accumulation on valves which might lead to malfunctionof the valves.

There are many known filter elements for use in a filter assembly in gassystems. Such filter elements generally comprise a cylindricalfiltration layer and a cylindrical anti-reentrainment barrier or a“drainage layer” surrounding the filtration layer on the outside of thefilter element. Such filter elements further comprise a bottom end caphaving a trough in which the coalescing and drainage layers areretained. Such filter elements are generally positioned to be verticalwhen in use. A filter element of this general kind is disclosed in GreatBritain Patent No. GB-A-2261830. A gas stream enters the tubular filterelement through an inlet port, and flows through the cylindrical wallsof the filter element, generally radially outward from the inside of thefilter element to the outside. When the assembly is used to collectliquid droplets in the gas stream (for example which is carried in thestream as an aerosol), the filtration layer will cause liquid dropletsto coalesce for collection. The coalesced liquid will be carried by theflow of gas to the drainage layer where the liquid can collect. Thedrainage layer is configured to ensure that re-entry of the liquid intothe gas stream is minimised. The liquid will sink to the bottom of thedrainage layer from which it can drain into a reservoir where it cancollect prior to disposal.

If the liquid does not drain from the drainage layer into the reservoirat least as quickly as liquid is supplied to the drainage layer, theliquid can tend to collect at the base of the drainage layer, where itforms a ‘wet band’ in which the material of the drainage layer issaturated with the liquid.

After prolonged use of the filter element, the depth of the wet band canincrease to the extent that gas flowing through the element wall willpass through the wet band. This can increase the resistance to flow ofgas through the filter element, and therefore give rise to an increasein pressure drop across the filter element. This can result in increasedoperating costs. Furthermore, gas flowing through or near a saturatedpart of the drainage layer can draw droplets of liquid from the drainagelayer into the gas flow, thereby re-contaminating the gas as it isdischarged from the filter element. It can therefore be seen thatsignificant accumulation of liquid in a wet band can reduce theoperating efficiency of the filter element. When the operatingefficiency is significantly reduced, the filter element in the filterassembly must be replaced, giving rise to expense and possibly also arequirement for the system in which the assembly is used to be shutdown.

The present invention provides an assembly in which the filtration layerin a filter element is compressed between an end cap on a filter elementand the wall of a housing in which the element is located when in use,by means of at least one longitudinally extending fin.

Accordingly, in one aspect, the invention provides a filter assembly forremoving material that is entrained in a gas stream, which comprises:

a filter element comprising a wall of a filtration medium which definesa hollow space, for a gas stream to flow from the space through the wallto be filtered, the wall including a filtration layer, and a drainagelayer located outside the filtration layer in which liquid separatedfrom the gas stream can collect, and first and second end caps atopposite ends of the wall, the first end cap including an inlet for agas stream to be supplied to the space within the wall, and

a housing in which the filter element can be received when in use, thehousing having an inlet port for a gas stream to flow into the housingwhich communicates with the inlet in the first end cap, and an outletport through which gas which has passed through the filtration mediumcan leave the housing,

in which the drainage layer extends over at least a part of the externalsurface of the second end cap between the second end cap and the housingand in which the part of the drainage layer between the second end capand the housing is locally compressed by at least one longitudinallyextending fin.

In another aspect, the invention provides a tubular filter element forcollecting material that is entrained in a gas stream, which has a wallof a filtration medium which defines a hollow space within it so that agas stream can flow through the wall to be filtered, the wall includinga filtration layer and a drainage layer located outside the filtrationlayer in which liquid separated from the gas stream can collect, theelement including first and second end caps at opposite ends of thewall, the first end cap including an inlet for a gas stream to besupplied to the space within the wall, the drainage layer extending overat least a part of the external surface of the second end cap, and inwhich the element includes a fin assembly with at least two finsextending along the filter element, externally of the drainage layer, toprovide localised compression of the drainage layer when the element islocated within a filter housing and the fins contact the internal wallof the housing. Features of the filter element of this aspect of theinvention are common to the filter element which forms part of theassembly of the invention.

The assembly of the invention has the advantage that liquid whichcollects in the drainage layer is encouraged to drain from it forcollection as a result of the localised compression by each fin. Liquidcan tend to drain from the drainage layer along each fin.

The effective engagement of the housing wall with the second end cap,through the fin and the drainage layer, can also help to locate thefilter element in the housing transversely. This can facilitate theformation of a reliable seal between the housing and the filter elementwhich might otherwise be disturbed if the element is able to movetransversely within the housing. The transverse location of the elementin the housing operates in conjunction with the axial location providedby the inter-engaging rib and groove.

The localised compression of the drainage layer by the fin needs to besufficient so that the surface tension of liquid in the drainage layeris broken, and therefore the liquid is encouraged to drain from thedrainage layer. Preferably, the thickness of the drainage layer isreduced in the region of localised compression by at least about 30%,more preferably, at least about 40%, especially at least about 50%.

Generally, the localised compression of the drainage layer will be notmore than about 90%, preferably not more than about 75%, especially notmore than about 60%.

Preferably, the angle between the fin and the axis of the housing is notmore than about 45°. When the fin extends at a non-zero angle to thehousing axis, the fins will extend helically relative to the axis of thehousing. Helical fins can have the advantage of providing contact withthe drainage layer around more of the periphery of the element. It willoften be preferred for the angle between the fins and the axis of thehousing to be less than 45°, for example not more than about 30°,preferably not more than about 20°, especially less than about 10°, forexample between 0° and about 5°. Drainage of liquid can be enhanced whenthe angle between the fins and the axis is small, especially when thefins extends approximately parallel to the axis of the housing, due togravitational forces acting on draining liquid being greater.

The fin can be directed inwardly and act against (and contact, directlyor indirectly) the external surface of the drainage layer. For example,the fin can be provided on the internal wall of the housing. The fin canbe formed integrally with the wall of the housing, for example as aresult of forming the housing by moulding (including casting).

The fin can be formed separately from the wall of the housing. This canbe advantageous as it can aid assembly of the housing, especially whenthe housing includes a separate drain outlet located at its bottom end.For example, the fin can be provided as a separate component which canbe placed in the housing. The separate fin element can be placed in thehousing so that it rests against the wall of the housing. A recess canbe provided in the wall of the housing to receive the separate filterelement. Preferably, the separate filter element has a stand parttowards its base which sits on the inner surface of the base of thehousing when the filter element placed in the housing. However, theseparate fin element having a stand part need not sit on the innersurface of the base of the housing. For example, the housing can beshaped and sized so that the separate filter element becomes wedged inthe housing at a desired site, as the separate fin element is slid intothe housing towards its site. In this arrangement preferably the standpart has a looped configuration, and the cross-sectional size of thehousing decreases continuously along its length.

The fin can be provided on the second end cap. The fin can have a basepart which connects the fin to the second end cap. When the elementincludes a flange for fastening the drainage layer (as discussed in moredetail below), it can be preferred for the base part to connect the finand the flange. The fin can be formed integrally with the second endcap, for example as a result of forming the housing by moulding(including casting). However, the fin can be formed separately from thehousing and be fastened to the second end cap. This can be advantageousas the fin can be fastened to the second end cap after the drainagelayer has been placed on the filter element. This can therefore easeassembly of the filter element.

A separate fin element can comprise a polymeric material. Preferredpolymeric materials include polyolefins (especially polyethylene andpolypropylene), polyesters, polyamides, polycarbonates and the like.Polymeric materials used for the separate fin component can bereinforced, for example by fibrous materials (especially glass fibres orcarbon fibres). Materials other than polymeric materials can be used,for example metals.

Preferably a separate fin element can be formed by moulding, forexample, by injection moulding.

Preferably, a separate fin element and the second end cap are formedfrom the same material.

The separate fin element can be fastened to the second end cap so thatit can be subsequently removed. For example, the separate fin elementcan be fastened to the second end cap through the use of a mechanicalfastening such as a latch, co-operating screw threads, or engagingbayonet formations. The separate filter element and second end cap canbe shaped and sized so that the separate fin element is fastened to thesecond end cap by the frictional force between them.

It can be advantageous to fasten the separate fin element to the secondend cap so that the separate fin element cannot subsequently be removed.In this case, preferably the separate fin element is fastened to thesecond end cap without the use of another material or component. Forexample, it can be preferred for the separate fin element to be fastenedto the second end cap through the use of a welding technique, forexample, ultrasonic or heat welding. However, it will be appreciatedthat the separate fin element can be fastened to the second end capusing another material such as a bonding agent, for example, anadhesive.

The fin can be directed outwardly, and act against (and contact,directly or indirectly) the internal surface of the drainage layer. Forexample, the fin can be provided on the second end cap of the element.The fin can be formed integrally with the element end cap, for exampleas a result of forming the housing by moulding (including casting).

Preferably, the drainage layer is compressed by at least twolongitudinally extending fins, more preferably by at least fourlongitudinally extending fins, especially preferably at least fivelongitudinally extending fins, for example by six longitudinallyextending fins. Preferably, the drainage layer is compressed by not morethan 10 longitudinally extending fins, more preferably not more than 8longitudinally extending fins, especially preferably not more than 7longitudinally extending fins. The number of fins can be dictated by thesize of the assembly. Typically, the larger the size of the filterassembly, the greater the number of fins.

Preferably, the fins are spaced apart around the filter elementapproximately uniformly. For example, when the filter element andhousing are circular in cross-section, and when there are three fins,they can be spaced apart about 120° around the housing, or when thereare four fins, they can be spaced apart about 90° around the housing.

Preferably, the edge of the fin which acts against the drainage layerdirectly or indirectly is generally rounded. This can provide linecontact with the drainage layer (subject to compression thereof) whichhas been found to encourage drainage of coalesced liquid from thedrainage layer.

Preferably, the fin is tapered, at least towards one end. This canfacilitate assembly of the filter assembly, when the filter element isintroduced into the housing.

Preferably, the length of the fin, measured from the second end of thefilter element towards the first end thereof, is at least about 1.0 cm,more preferably at least about 1.5 cm, especially at least about 2.0 cm.Preferably, the length of the fin, measured from the second end of thefilter element towards the first end thereof, is at most about 20.0 cm,more preferably at most about 15.0 cm, especially at most about 10.0 cm.When the fin is directed inwardly so that it acts against the externalsurface of the drainage layer, it is preferred that the fin extends inthe direction from the first end of the element towards the second end,to a point which is beyond the second end. This can encourage drainageof coalesced liquid from the drainage layer.

Preferably, the ratio of the length of the fin, measured from the secondend of the filter element towards the first end thereof to the length ofthe filter element is at least about 0.02, more preferably at leastabout 0.05, especially at least about 0.10, for example at least about0.5. This can help to ensure that the fin will extend over as much ofthe drainage layer as will be impregnated with coalesced liquid when theassembly is in use.

Preferably, the ratio of the length of the fin, measured from the secondend of the filter element towards the first end thereof to the length ofthe filter element is at most about 0.8, more preferably at most about0.7, especially at most about 0.6. This can help ensure that the findoes not extend over parts of the drainage layer that will unlikely beimpregnated with coalesced liquid when the assembly is in use. Further,this can help ensure that the fins not undesirably restrict the flow ofgas around the space between the filter element and the housing.

Preferably, the inside of the housing is approximately circular incross-section.

Preferably, the angle subtended at the centre of the housing by the oreach fin is not more than about 10°, more preferably not more than about5°, especially not more than about 3°.

Preferably, the transverse dimension of the element where the drainagelayer extends over the external surface of the second end cap is greaterthan the transverse dimension of the element between the first andsecond end caps. This can help to ensure that the drainage layer iscompressed locally at around the second end cap, between a fin which isdirected inwardly so that it acts against the external surface of thedrainage layer and the second end cap.

Preferably, the filter element has a substantially constantcross-sectional shape along its length. Preferably, the cross-section ofthe filter element is generally round. For example, the filter elementmay be circular or elliptical. However, it will be appreciated that thecross-section of the filter element need not be round. For example, thecross-section could be the shape of a square, triangle, or any otherregular or irregular shape.

The shape of the filter element when viewed along its axis (itscross-section shape) will generally be approximately constant over atleast most of the length of the element. However, it will be appreciatedthat its cross-section shape need not be constant. For example, thefilter element could be conical or pyramidal.

The cross-sections of the end caps and the wall at their interfacesshould be broadly the same so that the end caps can be fitted togetherwith the wall to provide fluid tight seals. Preferably, the filtrationand drainage layers are retained and sealed within a trough which isprovided in the first end cap.

The first end cap can include an inlet tube extending from the inlet inthe first end cap into the hollow space defined by the wall of thefilter element. The inlet tube can aid the even distribution of gasthrough the wall of the filter element. Preferably the inlet and theinlet tube are co-axial with the first end cap, however, it will beappreciated that this need not be the case.

Preferably, the tube is approximately straight, at least in the portionof its length that is within the hollow space. Preferably the ratio ofthe length of the tube (measured from the inner surface of the end capon which the tube is mounted) to the length of the filter element(measured between the inner surface of the opposite end caps) is atleast about 0.15, more preferably at least about 0.20, especially atleast about 0.25.

The first end cap can have an opening extending around the periphery ofthe inlet tube for supply of the gas stream to the element wall close tothe end cap. This can be advantageous as it can aid in the evendistribution of contaminant material across the element wall.

The inlet tube can be formed integrally with the first end cap, or canbe formed separately and subsequently fastened to the first end cap.

It can be advantageous to form the inlet tube separately as it allowsfor the production of a variety of different inlet tubes that can befastened to standard end caps. However, it can also be advantageous toform the first end cap and the inlet tube as one piece as this canreduce manufacturing costs. This can especially be the case if a varietyof different inlet tubes are not used in different applications.

If the inlet tube is formed separately from the first end cap, then theinterface between the first end cap and the inlet tube should form afluid tight seal. Preferably, the tube and the first end cap are formedform the same material. Preferably, the inlet tube can be fastened tothe first end cap so that it can be subsequently removed. For example,preferably the inlet tube is fastened to the first end cap through theuse of a mechanical fastening such as a latch, co-operating screwthreads, or engaging bayonet formations. More preferably, the inlet tubeand the first end cap are shaped and sized so that the inlet tube isheld within the first end cap by the frictional force between the inlettube and the first end cap.

It can be advantageous in some applications to fasten the inlet tube tothe first end cap so that the inlet tube cannot be subsequently removedfrom the first end cap. In this case, preferably the inlet tube isfastened to the first end cap without the use of a third party material.For example, preferably, the inlet port is fastened to the first end capthrough the use of a welding technique, for example, ultrasonic or heatwelding. However, it will be appreciated that the inlet port can befastened to the first end cap through the use of a third party materialsuch a bonding agent, for example an adhesive.

The inlet tube can have at least one helically extending rifle formationin its internal wall by which a helical flow is imparted to the gasstream when it leaves the tube. This can facilitate a more evendistribution of contaminant material in the gas stream over the lengthof the filter element. Furthermore, primary separation of liquiddroplets from the gas stream can be facilitated as a result of thehelical flow of gas entering the element. Details of a filter elementhaving an inlet tube with at least one helically extending rifleformation in its internal wall are disclosed in the co-pending PCTapplication filed with the present application which claims priorityfrom UK Patent Application No. 0417457.9. Subject matter that isdisclosed in that application is incorporated in the specification ofthe present application by this reference.

The inlet tube can have at least one opening in its side wall.Preferably, the inlet tube has an upstream band, a middlestream band anda downstream band, the said bands being axially adjacent to one anotherand non-overlapping, having equal axial lengths of at least 5% of thelength of the inlet tube, and being arranged such their planes areperpendicular to the axis of the inlet tube, and in which the proportionof the area of the side wall that is open in the upstream band issmaller than that in the middlestream band, and the proportion of thearea of the side wall that is open in the middlestream band is smallerthan that in the downstream band. This can help ensure that the supplyof gas into the filter element is graded, which can lead to a more evendistribution of gas flowing through the element wall. Details of afilter element having an inlet tube with at least one opening in itsside wall are disclosed in the co-pending PCT application filed with thepresent application which claims priority from UK Patent Application No.0417462.9. Subject matter that is disclosed in that application isincorporated in the specification of the present application by thisreference.

The filtration medium should generally be sealed to the second end capso that there is no path for gas to flow past the filtration medium.Preferably, the filtration layer is retained and sealed within a troughwhich is provided in the second end cap. Preferably, the drainage layerextends over the outermost edge of the second end cap and is sealed tothe undersurface of the second end cap.

The filtration medium (or one or more layers thereof) can be sealed tothe end caps using a quantity of a bonding material such as an adhesive.The bonding material should be selected according to fluids with whichthe element will come into contact when in use so that there are noadverse reactions between the bonding material and the fluids. Thefiltration medium might be sealed to the end caps using other techniquessuch as welding, for example by localised application of heat, or byultrasonic welding.

The drainage medium (or one or more layers thereof) can be sealed to thefirst end cap using a quantity of a bonding material such as anadhesive. The bonding material should be selected according to fluidswith which the element will come into contact when in use so that thereare no adverse reactions between the bonding material and the fluids.The drainage medium might be sealed to the first end cap using othertechniques such as welding, for example by localised application ofheat, or by ultrasonic welding.

Preferably one or each of the end caps is formed from a polymericmaterial. Preferred polymeric materials include polyolefins (especiallypolyethylene and polypropylene), polyesters, polyamides, polycarbonatesand the like. Polymeric materials used for the end caps can bereinforced, for example by fibrous materials (especially glass fibres orcarbon fibres). Materials other than polymeric materials can be used,for example metals. The first and second end caps will generally beformed from the same material or materials.

Preferably one or each of the end caps is formed by moulding, forexample, by injection moulding.

Preferably, the first end cap or the second end cap (or each of the endcaps) has means for locating the filter element in the housing of thefilter assembly, especially to control the alignment of the element inthe housing. For example, the first end cap can have at least one ribwhich is received in an appropriate groove in a housing. This can alsofacilitate the loosening of the filter element from the housing headpart when the housing body is rotated relative to the housing head part.Details of a filter assembly in which the first end cap has at least onerib that is received within an appropriate groove in the housing aredisclosed in the co-pending PCT application filed with the presentapplication which claims priority from UK Patent Application No.0417463.7 and 0428567.2. Subject matter that is disclosed in thatapplication is incorporated in the specification of the presentapplication by this reference.

The inlet at the first end cap of the filter element should be capableof coupling with a part in a housing for the element so that a tightseal is formed. This can ensure that all gas that enters the inlet ofthe filter assembly, enters the filter element. Techniques for sealinginlet ports are known, for example as disclosed in PCT Application No.WO-A-99/30798.

The second end cap can provide a trough in which the filtration layer isretained, and the drainage layer can be wrapped around the outside ofthe end cap.

Preferably, the drainage layer extends from the wall over at least apart of the external surface of the second end cap and over the face ofthe second end cap which faces away from the first end cap. Preferably,the drainage layer extends over at least 75% of the area of the face ofthe second end cap which faces away from the first end cap. Morepreferably, the drainage layer extends over at least 80% of the area ofthe face of the second end cap which faces away from the first end cap.Especially preferably, the drainage layer extends over at least 85% ofthe area of the face of the second end cap which faces away from thefirst end cap.

The filter element can include a clamp which can be fastened to thesecond end cap with the drainage layer clamped between the clamp and theend cap. The clamp can include a pin which can penetrate the end cap toform the connection between them. The clamp can be provided with athreaded stud which can be received in a threaded bore in the end cap.The clamp can provide a threaded bore, in which a threaded stud on theend cap can be received. Alternatively, the clamp and the end cap can befastened together by other press-fit arrangements, for example by apress-stud arrangement or the like.

The clamp can be made from polymeric materials or from metallicmaterials. It should have sufficient rigidity to ensure that thedrainage layer is retained in place. Metallic materials can often bepreferred for this application. Suitable materials should not have anyadverse reaction with fluids with which the element will come intocontact when in use.

Preferably, the second end cap includes a flange which defines anannular slot in which the drainage layer can be inserted to retain it inplace over the external surface of the end cap. The filter element caninclude an elastic ring member which can be fitted into the annularslot, over the drainage layer, to retain the drainage layer in the slot.Preferably, the surface of the flange remote from the end cap willgenerally be approximately planar, extending perpendicular to the axisof the element. Preferably, the surface of the flange which defines theslot is roughened, textured or otherwise featured so that the drainagelayer is engaged within the slot.

Preferably, the slot is tapered inwardly from its peripheral edgetowards the centre, so that the material of the drainage layer iscompressed progressively as it is inserted into the slot. The slot istherefore generally V-shaped when the end cap is viewed from one side.

The flange can be spaced from the second end cap by a stem extendingbetween them. More than one stem can be provided. Preferably, only asingle stem is provided. Preferably, the flange and the stem areco-axial. Preferably, the ratio of the transverse dimension of the stemto the transverse dimension of the flange not more than about 0.5. Morepreferably, the ratio of the transverse dimension of the stem to thetransverse dimension of the flange is at least about 0.25. The smallerthe transverse dimension of the stem with respect to the transversedimension of the flange, the larger the area between the second end capand the flange in which the drainage layer can be received. This canallow for a larger volume of drainage layer to be folded under the faceof the second end cap. Accordingly, the provision of a smaller stem canreduce the tendency for the wet band to build up in the flow path of agas.

The transverse dimension of the flange (which will be its diameter if itis circular) can be the same as or larger than that of the part of thesecond end cap which closes the hollow space. However, preferably, thetransverse dimension of a flange is smaller than that of the second endcap. Preferably, the ratio of the transverse dimension of the flange tothe transverse dimension of the second end cap is not more than about0.8, more preferably, not more than about 0.5. This can help to minimiseobstructions to drainage of collected liquid from the drainage layer.

The drainage layer can be held within the annular space defined by theflange without the use of another component. For example, the second endcap and flange can be configured so that drainage layer can be held bythe friction forces between the drainage layer and the flange and secondend cap. Further, the drainage layer can be held within the annularspace such as welding, for example by localised application of heat, orby ultrasonic welding.

Preferably, the flange is of sufficient size that the drainage layerthat the drainage layer can be received under it, between it and thesecond end cap, and can be held in position through use of anothercomponent. This can allow for the drainage layer to be removed fromwithin the annular slot. Preferably, the flange is provided on a stem.The length of the stem defines the width of the slot and at its innerend. Preferably, the other component is a ring member, especially anelastic ring member, which can be fitted into the annular slot, over thedrainage layer, to retain the drainage layer in the slot. The othercomponent can hold the drainage layer against the stem. Preferably, thethickness of the ring member is such that it is an interference fit inthe slot when the slot contains the drainage layer, especially when theslot tapers inwardly from its peripheral edge towards the centre. Whenthe ring has elastic properties, it is preferred that it is held tensionas it tries to contract to a smaller diameter, as a result of thefrictional forces between the drainage layer and one of the walls of theslot.

Preferably, the flange is of sufficient size that the drainage layerthat the drainage layer can be received under it, between it and thesecond end cap, and can be held in position through use of anothercomponent. This can allow for the drainage layer to be removed fromwithin the annular slot. Preferably, the flange is provided on a stem.The length of the stem defines the width of the slot and at its innerend. Preferably, the other component is a ring member, especially anelastic ring member, which can be fitted into the annular slot, over thedrainage layer, to retain the drainage layer in the slot. The othercomponent can hold the drainage layer against the stem. Preferably, thethickness of the ring member is such that it is an interference fit inthe slot when the slot contains the drainage layer, especially when theslot tapers inwardly from its peripheral edge towards the centre. Whenthe ring has elastic properties, it is preferred that it is held tensionas it tries to contract to a smaller diameter, as a result of thefrictional forces between the drainage layer and one of the walls of theslot.

Suitable materials for an elastic ring member include elastomericpolymeric materials, for example certain rubbers. The ring member mightbe formed with suitable resilient characteristics from a metal, forexample as a helical spring.

Preferably, the flange on the second end cap has at least one apertureformed in it through which liquid within the drainage layer can drain.

The edge of the flange on the second end cap which faces the drainagelayer when inserted into the annular space can be rounded. This can helpto minimise undesirable stress concentrations on the drainage layer, andcan minimise obstruction to drainage of collected liquid from thedrainage layer.

The surface of the flange which faces the slot can have at least oneprotrusion formed in it, directed into the annular slot. This canprovide localised compression of the part of the drainage layer where itis inserted into the slot. This localised compression can encouragedrainage of liquid from the drainage layer. There can be more than oneprotrusion. Preferably, the or each protrusion extends to the peripheryof the flange. Protrusions can be in the form of number of raisedcastellation-like formations arranged around the perimeter of theflange.

Preferably, the second end cap includes an upstand portion which extendstowards the first end cap, into the space within the filter element.This can help to encourage fluid to flow transversely towards the wallof the element. Preferably, the upstand portion is located centrallywithin the element.

Preferably, the upstand portion of the second end cap tapers inwardlytowards the end thereof which is closer to the first end cap. Theupstand portion can have a substantially constant cross-sectional shapealong its length. Preferably, the cross-section of the upstand portionis generally round, for example elliptical or especially circular.

When the second end cap includes an upstand portion that has a circularcross-section, and when the second end cap includes a flange and a stem,preferably the stem and the upstand portion are co-axial, and preferablythe diameter of the upstand portion is the same as that of the stem.This can ease construction of the second end cap, especially when thesecond end cap is formed by moulding.

Materials suitable for use in a filtration filter element will beselected according to the nature of the gas that is being filtered, thenature of the contaminants (liquid droplets, aerosols, solid particlesetc) to be filtered from the gas, the pressure differential across thefilter and so on. Such materials are known, including those used byDomnick Hunter Limited in products which are available under the trademark OIL-X. Suitable materials include, borosilicate and other glassfibres, activated carbon minerals, activated silica materials and so on.A filtration layer can be made from woven fibres. However, as will beappreciated, a filtration layer can be made from sheets of non-wovenfibres. For example, a microfiber filtration layer made from fineorganic or inorganic fibres is preferred. Preferably, a coarser fibrelayer is fitted on the inside of a microfiber filtration layer. Thiscoarser layer can protect a microfiber filtration layer from grosspollution.

Preferably, the filtration layer can comprise a layer of a materialwhich has been folded so that it is fluted (or pleated). This canincrease the surface area of the filtration layer through which airflowing through the filter element will pass. This can also help toincrease the rigidity of the filtration layer.

The element can include at least one support for the filtration layer.This can help to retain the filtration layer in its position within thefilter element. This can also increase the rigidity of the filterelement. A support can be provided within the hollow space, positionedagainst the internal surface of the filtration layer. A support can bepositioned outside the filtration layer, for example between thefiltration layer and the drainage layer. Preferably, a first supportmade of a rigid material is positioned within the hollow space againstthe internal surface of the filtration layer, and a second support madeof rigid material is positioned outside the filtration layer.Preferably, the or each support is perforated to allow a gas stream toflow therethrough. The material for the support should have sufficientrigidity to withstand the forces to which the element is exposed, duringassembly of the element and an assembly containing the element, andduring use. The material can be metallic, for example a stainless steel.

The drainage layer of the filter element should comprise a material thatis capable of retaining liquid that has been coalesced by the filtrationlayer, and is carried to the drainage layer by a gas stream that flowsthrough the drainage layer. The drainage layer of the filter elementwill generally be porous, and made from a material which encourages flowof coalesced liquid towards the base of the filter element. Factorsaffecting the drainage characteristics include pore size and structure,and the material of the drainage layer, including for example thesurface energy of liquid which is in contact with the material.Materials suitable for use in the drainage layer are used in similarproducts sold by Domnick Hunter Limited under the trade mark OIL-X.Suitable materials include open-celled foam plastics, felted fabricmaterial, and expanded foam materials.

Generally, the filter element will be arranged in use with the first endcap located above the second end cap, when the first end cap might bereferred to as the top end cap, and the second end cap referred to asthe bottom end cap.

The housing can comprise a head and a body. The head and the body can beseparable, providing access to the interior of the housing, especiallyfor replacement of the filter element. The head and body should becapable of being connected to one another to form a fluid tight seal,for example by means of cooperating screw threads or by means ofcooperating bayonet formations.

The housing should provide an inlet port for a gas stream to flow intothe housing, and an outlet port through which gas which has passedthrough the filtration medium can leave the housing. The ports willgenerally be provided in the housing head.

Preferably, the housing includes a reservoir in which coalesced liquidwhich drains from the drainage layer can collect. The reservoir can beprovided by a space within the housing below the filter element.

Preferably, the housing includes a drain outlet for coalesced liquidwhich drains from the drainage layer. The outlet will generally providefor removal of liquid which has collected in a reservoir. The drainshould preferably be capable of opening without depressurising thehousing. A suitable drain mechanism is disclosed in European Patent No.EP-A-81826.

The housing should be formed from a material which is capable ofwithstanding the internal pressures to which it is subjected when inuse. Metals will often be preferred, for example aluminium and alloysthereof, and certain steels.

The inlet at the first end cap of the filter element should be capableof coupling with a part in a housing for the element so that a tightseal is formed. This can ensure that all gas that enters the inlet ofthe filter assembly, enters the filter element. Techniques for sealinginlet ports are known, for example as disclosed in PCT Application No.WO-A-99/30798.

Preferably, the parts of the housing fit together for the purposes ofconnection in such a way that the body part as a male part fits withinthe head part as a female part.

Preferably, the inlet on the end cap of the filter element has acompressible O-ring seal for forming a fluid tight seal with the headpart of the housing.

A flow conduit having a first conduit opening for communication with aport (generally the inlet port in the housing head) for the gas that isto be filtered and a second conduit opening for communication with theinlet on the first end cap, can be provided. Generally, the flow of thegas stream towards and away from the assembly will be horizontal. Thefilter element will generally be arranged vertically so that the housinghead is at the top of the housing with the filter element locateddepending below it. In such constructions, the axis of the first conduitopening and the axis of the second conduit opening will not be aligned.Generally, the axis of the first conduit opening will be substantiallyperpendicular to the axis of the second conduit opening.

Preferably, the flow conduit will be configured to provide a smooth flowpath for a gas which flows between the first and second conduitopenings. The smooth flow path can be constructed to reduce restrictionof the flow of the gas stream compared for example with a flow conduitwhich presents a discontinuous flow path which is sharply angled orcontains steps or other obstructions.

The flow conduit can be provided as part of the first end cap. However,preferably the flow conduit is formed separately from the first end capand subsequently fastened to the first end cap. This can reduce thecosts of manufacturing the first end cap, especially when the first endcap and flow conduit and formed by a moulding process. The interfacebetween the inlet in the filter element and the flow conduit should forma fluid tight seal to ensure that all gas that flows through the flowconduit enters the filter element. Preferably, the flow conduit and thefirst end cap are formed form the same material. Preferably, the flowconduit can be fastened to the first end cap so that it can besubsequently removed. For example, preferably the flow conduit isfastened to the first end cap through the use of a mechanical fasteningsuch as a latch, co-operating screw threads, or engaging bayonetformations. More preferably, the flow conduit and the inlet in the firstend cap are shaped and sized so that the flow conduit is held within theend cap by the friction forces between the flow conduit and the firstend cap.

Preferably, the flow conduit extends into the head part of the housingwhen the assembly is assembled. A seal can be provided in one or both ofthe surfaces of the housing and the flow conduit which contact oneanother. For example, a seal can be provided in a face of the flowconduit around the first conduit opening. The seal can be provided in agroove in that face. It can be provided as a separable component of theflow conduit. It might be formed as an integral part of the flowconduit, for example as a result of being formed by moulding in place.The material for seals in an assembly according to the invention will beselected according to the application for the assembly; the seal willgenerally be provided by an elastomeric material.

The head part can present a bore in which the end of the flow conduit isreceived. Preferably, the head part of the housing has internal wallswhich define a primary chamber within it which communicates with one ofthe ports and also with the hollow space within the tubular filterelement. Generally, the axis of the port and the axis of the tubularfilter element will not be aligned. Preferably, a seal is providedbetween the internal wall of the primary chamber at or towards the freeend thereof and the flow conduit at the second conduit opening thereof.Preferably, the flow conduit extends from the hollow space into theprimary chamber. It has been found the provision of a flow conduit thatextends from the hollow space into the primary chamber can give rise tosignificant advantages, including that any turbulence in the flow of gaswithin the chamber between the port and the hollow space resulting fromthe non-alignment of the axes can be reduced. Further, such an assemblyhas fewer constraints on the design of the head part with a view tominimising flow resistance compared with other assemblies, such as thatdisclosed in PCT Application No. WO-02/38247. Details of a filterassembly having a primary chamber into which the flow conduit extendsare disclosed in the co-pending PCT application filed with the presentapplication which claims priority from UK Patent Application No.0417458.7. Subject matter that is disclosed in that application isincorporated in the specification of the present application by thisreference.

The flow conduit can be made with additional features during itsmanufacture. For example, a port might be formed in it for connection tomeans for indicating the pressure within the conduit. Such ports areknown, for example as disclosed in PCT Application No. WO-A-99/30798.Details of an alternative arrangement for such a port are disclosed inco-pending application entitled Filter Assembly which is filed with thepresent application bearing agent's reference P205958. Subject matterthat is disclosed in that application is incorporated in thespecification of the present application by this reference.

The flow conduit can contain at least one vane positioned within it sothat the flow of gas along the conduit between the first and secondconduit openings passes over the vane and is smoothed by it. Theprovision of a vane in the flow conduit has been found to reduce theresistance to flow of gas through the flow conduit compared with a flowconduit which does not include a vane. This can enable the efficiency ofa filter element to be enhanced compared with known assemblies of thisgeneral kind. Details of a filter assembly having a flow conduit whichhas a vane are disclosed in the co-pending PCT application filed withthe present application which claims priority from UK Patent ApplicationNo. 0417464.5. Subject matter that is disclosed in that application isincorporated in the specification of the present application by thisreference.

Preferably, the flow conduit contains at least two of the said vanes.

The flow conduit can be formed in first and second matable pieces, inwhich the first piece comprises a first part of the flow conduit walland the vane, and the second piece comprises a second part of the flowconduit wall which has a recess formed in it in which the end of thevane that is remote from the first part of the conduit wall can bereceived when the first and second pieces are mated. Preferably, thefirst and second matable pieces of the flow conduit mate in a planewhich contains the first and second conduit openings. Details of afilter assembly in which the flow conduit can be formed in first andsecond matable pieces are disclosed in the co-pending PCT applicationfiled with the present application which claims priority from UK PatentApplication No. 0417464.5. Subject matter that is disclosed in thatapplication is incorporated in the specification of the presentapplication by this reference.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, and with reference to the accompanying drawings, in which:

FIG. 1 is sectional side elevation through a filter assembly accordingto the present invention which comprises a filter element and thehousing in which the element is located when in use.

FIG. 2 is a sectional side elevation through a filter element accordingto the present invention.

FIG. 3 is a sectional side elevation of the top end cap of the filterelement shown in FIG. 2.

FIG. 4 is a schematic perspective view of the top end cap of the filterelement shown in FIG. 2.

FIG. 5 is a schematic top view of the top end cap of the filter elementshown in FIG. 2.

FIG. 6 is a schematic sectional elevation of the top end cap of thefilter element shown in FIG. 2 illustrating a second embodiment of theinlet tube of the filter element.

FIG. 7 is a schematic sectional elevation of the top end cap of thefilter element shown in FIG. 2 illustrating a third embodiment of theinlet tube of the filter element.

FIG. 8 is a schematic sectional elevation of the top end cap of thefilter element shown in FIG. 2 illustrating a fourth embodiment of theinlet tube of the filter element.

FIG. 9 is a schematic sectional elevation of the top end cap of thefilter element shown in FIG. 2 illustrating a fifth embodiment of theinlet tube of the filter element.

FIG. 10 is a schematic sectional elevation of the top end cap of thefilter element shown in FIG. 2 illustrating a sixth embodiment of theinlet tube of the filter element.

FIG. 11 is a sectional elevation of the first vane of the filter elementshown in FIG. 2.

FIG. 12 is a sectional elevation of the first vane of the filter elementshown in FIG. 2 according to a second embodiment.

FIG. 13 is a sectional elevation of the first vane of the filter elementshown in FIG. 2 according to a third embodiment.

FIG. 14 is a sectional elevation of the first vane of the filter elementshown in FIG. 2 according to a fourth embodiment.

FIG. 15 is a sectional elevation view of a flow conduit according to theinvention illustrating the calculation of the position of the firstvane.

FIG. 16 is a sectional elevation view of the flow conduit shown in FIG.15 illustrating the calculation of the gap/chord ratio of the vanes.

FIG. 17 is a sectional elevation view of the flow conduit shown in FIG.15 illustrating the calculation of the radius ratio and the aspect ratioof the sub-flow conduits defined by the vanes.

FIG. 18 a is a schematic perspective view of the first piece of a twopiece flow conduit according to the invention.

FIG. 18 b is a schematic perspective view of the second piece of the twopiece flow conduit shown in FIG. 18 a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, FIG. 2 shows a filter element which comprisesa cylindrical wall section 2 formed from a filter medium, and top andbottom end caps 4 and 6.

The wall section 2 defines a hollow space 8 within it. The filter mediumof the wall 2 comprises a cylindrical filtration layer 10 and acylindrical anti-reentrainment layer or drainage layer 12 which fitssnugly around the filtration layer on the outside of the filter element.

The top end cap 4 contains a flow conduit 34 which defines a flow path36 for gas which is to be filtered. The flow conduit 34 has a port 80 init for connection to a gauge for measuring the differential pressureacross the filter element. When the filter element is located within ahousing (described in more detail below) the port 80 can be received ina downwardly facing socket in the housing head, forming a seal bycompression of an O-ring between the external surface of the port andthe internal surface of the socket.

The flow conduit 34 has a first opening 162 having a first axis A, and asecond opening 164 having a second axis B. The angle between the axes A,B of the first 162 and second 164 openings is 90°. The flow conduit 34provides a continuous flow path between the two openings, and thereforeprovides a smooth change of direction for gas flowing therethrough whenin use. The flow conduit 34 turns about an axis D which extendsperpendicularly to the axes A, B of the first 162 and second 164 conduitopenings. (As shown in FIG. 2, the axis D about which the flow conduit34 turns extends perpendicularly to the plane along which thecross-section of FIG. 2 of the filter element is taken).

The flow conduit 34 contains first 158, second 159 and third 160 curvedvanes extending perpendicularly across the flow conduit. Each vanecurves around its own axis and the radius of curvature is the same foreach vane. Further, the length of the vanes, measured between theirleading and trailing edges, is the same for each vane. The axes aroundwhich the vanes curve extend perpendicularly to the axes A, B of thefirst and second conduit openings 162, 164. For example, as shown inFIG. 2, the second vane 159 curves around an axis C which extendsparallel to the axis D about which the flow conduit 34 turns. The vaneseach have concave 168 and convex 170 surfaces, wherein the concavesurface of each vane faces the first 162 and second 164 openings of theflow conduit 34. Therefore, the vanes help guide the flow of gas betweenthe first 162 and second 164 openings.

The shape and configuration of the first 158, second 159 and third 160vanes is identical and shown in more detail with reference to the firstvane 158 in figure 11. As shown, the first vane 158 has a roundedleading edge 172 which faces into the direction of the flow of gas 176when the filter element is in use, and a trailing edge 174. Thethickness of the first vane 158 is substantially constant between itsleading and trailing edges. The vane has a straight portion 182 proximalits trailing edge 174, and a curved portion 186 extending between itsleading edge and the straight portion. The length of the straightportion 182 is 5% of the total length of the vane 158 between itsleading 172 and trailing 174 edges. The angle of incidence of the vaneto the flow of gas when in use (i.e. the angle 180 between a straightline 176 projecting parallel to the direction of the flow of gasimmediately upstream of the vane 158 and a straight line 178 projectingtangentially from the convex surface 170 of the vane at its leading edge172) is 4°.

The first 158, second 159 and third 160 vanes need not have the shapeand configuration of the vane shown in FIG. 11. For example, the vanescan have the shape and configuration of an elbow shaped vane 188 asshown in FIG. 12. The elbow shaped vane 188 is substantially similar inconfiguration to that shown in FIG. 11 and like parts share likereference numerals. However, the elbow vane 188 has a straight leadingportion 190 proximal its leading edge 172, as well as a straighttrailing portion 192 proximal its trailing edge 174 and a curved portion194 between them.

Further, the first 158, second 159 and third 160 vanes can have theshape and configuration of the crescent shaped vane 196 shown in FIG.13. The crescent shaped vane 196 is substantially similar inconfiguration to that shown in FIG. 11 and like parts share likereference numerals. However, the thickness of the crescent shaped vane196 is not uniform between its leading 172 and trailing edges 174.Instead, the thickness of the crescent shaped vane 196 continuouslyincreases as you travel from its leading edge 172 towards the midpointbetween the leading and trailing 174 edges, and continuously decreasesas you travel from the midpoint to the trailing edge.

Further still, the first 158, second 159 and third 160 vanes can havethe shape and configuration of the curved-aerofoil shaped vane 198 shownin FIG. 14. The curved-aerofoil shaped vane 198 is similar inconfiguration to that shown in FIG. 11 and like parts share likereference numerals. However, the thickness of the curved-aerofoil shapedvane 196 is not uniform between its leading 172 and trailing 174 edgesand is generally thicker towards its leading edge than towards itstrailing edge.

Referring now back to FIG. 2, the gap/chord ratio of the first 158,second 159 and third 160 vanes shown in FIG. 2, is 0.45.

The first 158, second 159 and third 160 vanes are arranged such thatradius ratio of all of the sub-flow conduits (described in more detailbelow in relation to FIG. 17) is at least 1.

The first 158, second 159 and third 160 vanes are arranged such thataspect ratio for all the sub-flow conduits is at least 1.

FIGS. 15 to 17 illustrate how the preferred distance between the concavesurface of the inner most vane and wall of the flow conduit the concavesurface of the vane faces, the gap/chord ratio, and the radius ratio andaspect ratio for a flow conduit with vanes, can be calculated. For sakeof simplicity of illustration, the flow conduits 34 of FIGS. 15 to 17contain only a first vane 232 and a second vane 234. Otherwise, allother parts of the flow conduit 34 of FIG. 15 are the same as that shownin FIG. 2 and share the same reference numerals.

As illustrated in relation to FIG. 15, the preferred positioning of theinner most vane (i.e. the first vane 232) can be calculated so that thedistance 200 between its concave surface 168 and wall of the flowconduit the concave surface of the vane faces, is equal to: R-r cos θ/2,where (R) is the radius of the curvature 202 of the centre line 204 ofthe turn of the flow conduit 34, (r) is the radius of the curvature 206of the first vane 232, and θ is the angle between the axes of the first162 and second 164 conduit openings (i.e. 90° as shown in FIGS. 2 and15).

As illustrated in relation to FIG. 16, the gap/chord ratio is calculatedas the ratio of the distance 208 between the midpoints of the first 232and second 234 vanes, and the chords 210 of the vanes.

As illustrated in relation to FIG. 17, the first 232 and second 234vanes divide the flow conduit 34 into first, second and third sub-flowconduits having first 212, second 214 and third 216 centre line of turnsrespectively. The radius ratio of the second sub-flow conduit having acentre line of turn 214, is calculated as the ratio of the radius of thecurvature 218 of the centre line of the turn 214 of the second sub-flowconduit to the width 208 of the sub-flow conduit measured between thefirst 232 and second 233 vanes.

Still referring to FIG. 17, the aspect ratio of the second sub-flowconduit having a centre line of turn 214 is calculated as the ratio ofthe depth of the second sub-flow conduit to the width 208 of the secondsub-flow conduit measured between the vanes. As shown in FIG. 4, thedepth of a sub-flow conduit is the distance 220 between the opposingwalls of the flow conduit 34 between which the vanes that define thesub-flow conduit extend.

Referring now to FIG. 2, the flow conduit 34 and the top end cap 4 ofthe filter element are formed as separate pieces. Further, the flowconduit 34 is formed from two pieces.

A two piece flow conduit 34 is shown in FIGS. 18 a and 18 b. For sake ofsimplicity, the flow conduit 34 of FIGS. 18 a and 18 b contains only afirst vane 232 and a second vane 234. Otherwise, all other parts of theflow conduit 34 of FIG. 15 are the same as that shown in FIG. 2 andshare the same reference numerals. With reference to FIGS. 18 a and 18b, the flow conduit 34 comprises first 221 and second 222 matablepieces. The first 221 and second 222 pieces are mirror images of eachother (the plane of symmetry of the pieces extending through both theaxes of the first 162 and second 164 conduit openings) except that thefirst 232 and second 234 vanes extend from, and are part of, the firstpiece 221 of the flow conduit 34. The first 232 and second 234 vaneshave respective tangs 224, 226 at their free ends (i.e. the ends distalto the wall of the first piece 220 from which the vanes extend) that canbe received within corresponding recesses 228, 230 formed in the wall ofthe second piece 222 of the flow conduit 34. Therefore, when the first221 and second 222 pieces are brought together to form the flow conduit34, the tangs 224, 226 are received within the corresponding recesses228, 230 so that the first 232 and second 234 vanes are secured to thesecond piece 222 at their free ends.

Referring back to FIG. 2, the top end cap 4 further comprises an inlettube 106, co-axial with the top end cap 4, which extends from the secondconduit opening 164 of the flow conduit 34 part way into the hollowspace 8 to a downstream open end. In this embodiment, the inlet tube 106and top end cap 4 are one piece. However, it will be appreciated thatthey need not be one piece. The inlet tube 106 defines a flow path 108for gas which is to be filtered and is in fluid connection with the flowpath 36 of the top end cap 4.

The top end cap 4 has annular opening 112 extending around the inlettube 106 proximal to the flow conduit 34. The inlet tube 106 is heldwithin the annular 112 opening by fins 114 that extend between the topend cap 4 and the inlet tube 106.

As best shown in FIG. 3, the inlet tube 106 has a rifling formationdefined on its inner wall, extending from its end proximal to the topend cap 4, part way towards its end distal to the top end cap. Therifling formation is provided by a helically extending ridge 110 on theinner wall of the inlet tube 106.

Referring now to FIG. 4, a schematic perspective view of the top end cap4 of FIG. 2 is shown. For simplicity and to enable illustration of thehelically extending groove 110, the first vane 158 is not shown. Asshown, the inlet tube 106 has first 116, second 118, third 119 andfourth (not shown) openings in its side wall. The openings are in theform of slits that taper uniformly from an open end at the downstreamopen end of the inlet tube 106, to a point towards the upstream open endof the inlet tube. Therefore, the proportion of the side wall of theinlet tube 106 that is open, increases towards the downstream open endof the inlet tube. The slits extend along 50% of the length of the inlettube and in a direction parallel to the axis of the inlet tube. Theslits are positioned equally around the inlet tube 106, i.e. they arespaced apart from each other by about 90° around the inlet tube.

FIGS. 6 to 10 show alternative embodiments of inlet tubes according tothe invention that can be used with the top end cap 4 of FIG. 2. The topend caps 4 shown in FIGS. 6 to 10 are the same as that shown in FIG. 2.However, for simplicity, the first 158, second 159 and third 160 vanes,and also the helically extending ridge 110, are not shown.

FIG. 6 shows an inlet tube 120 according to the invention which issimilar in configuration to that shown in FIG. 2, except that the first,second, third and fourth slits taper to a flat end 122, instead of to apoint.

FIG. 7 shows an inlet tube 124 which is similar in configuration to thatshown in FIG. 2, except that the inlet tube 124 only has one opening126, which is in the form of a helically extending slit. The helicallyextending slit extends from an open end at the downstream open end ofthe inlet tube 124, to a point towards the upstream open end of theinlet tube. As shown, the angle between the plane perpendicular to theaxis of the inlet tube 124 and a straight line projecting substantiallyalong a portion of the slit 126 is constant. However, the width of thehelically extending slit 126, taken perpendicular to the straight lineprojecting substantially along a portion of the slit, decreases towardsthe upstream open end. Therefore, the proportion of the side wall of theinlet tube 124 that is open, increases towards the downstream open endof the inlet tube.

FIG. 8 shows an inlet tube 128 which is similar in configuration to thatshown in FIG. 7 in that the inlet tube 128 only has one opening 130 inthe form of a helically extending slit. However, in contrast to theinlet tube 124 shown in FIG. 7, the width of the slit 130 shown in FIG.8 in constant along its entire length. Further, the angle between theplane perpendicular to the axis of the inlet tube 128 and a straightline projecting substantially along a portion of the slit 130 decreasestowards the downstream open end of the inlet tube. Therefore, theproportion of the side wall of the inlet tube 128 that is open,increases towards the downstream open end of the inlet tube.

FIG. 9 shows an inlet tube 132 which is similar in configuration to thatshown in FIG. 2, except that the side wall has a plurality of openings134 in the form of apertures. The apertures can be identified as whollyfalling within three bands (an upstream band, designated generally by136, a middlestream band 138, and a downstream band 140), extendingaround the inlet tube, each band defining a plane perpendicular to theaxis of the inlet tube. The upstream band 136 is located within thedownstream half of the inlet tube 132. Four apertures (only one of whichcan be seen in FIG. 9) are present in the upstream band 136, eightapertures (only two of which can be seen in FIG. 9) are present in themiddlestream band 138, and twelve apertures (only three of which can beseen in FIG. 9) are present in the downstream band 140. The apertureshave two equal length, straight, parallel sides, and has a convex end ateach end of the aperture that extends between the parallel sides. Thelength of the parallel sides is longer than the transverse distancebetween them. All of the apertures 134 are equal in shape and dimension.

The four apertures in the upstream band 136 are positioned equallyaround the inlet tube 132, i.e. they are spaced apart from each other byabout 90° around the inlet tube. The eight apertures in the middlestreamband 138 are divided into four sets of two apertures. The four sets oftwo apertures are positioned equally around the inlet tube 132, i.e.they are spaced apart from each other by about 90° around the inlettube. The twelve apertures in the downstream band 140 are divided intofour sets of three apertures. The four sets of three apertures arepositioned equally around the inlet tube 132, i.e. they are spaced apartfrom each other by about 90° around the inlet tube. Therefore, theproportion of the side wall of the inlet tube 132 that is open increasestowards the downstream open end of the inlet tube.

FIG. 10 shows an inlet tube 142 which is similar in configuration tothat shown in FIG. 9, except that the apertures 152 are circular inshape, and that four bands (an upstream band, designated generally by144, an upstream middlestream band 146, a downstream middlestream band148, and a downstream band 150) can be identified instead of three. Fourapertures (only one of which can be seen in FIG. 10) are present in theupstream band 144, eight apertures (only two of which can be seen inFIG. 10) are present in the upstream middlestream band 146, twelveapertures (only three of which can be seen in FIG. 10) are present inthe downstream middlestream band 148, and sixteen apertures (only fourof which can be seen in FIG. 10) are present in the downstream band 150.

Referring back to FIG. 2, each of the end caps has a trough 14 formed init. The top part of the filtration layer 10 and the drainage layer 12are retained and sealed in the trough 14 of the top end cap 4, and thebottom part of the filtration layer is retained and sealed in the trough14 of the bottom end cap 6.

The bottom end cap 6 further comprises a flange part 16, spaced apartfrom the surface 20 of the second end cap facing away from the top endcap 4. The flange part 16 extends generally transverse to the axis ofthe filter element. The flange part 16 is located centrally with respectto the bottom end cap 6, and is spaced from it by a co-axial stem 28extending between them. The flange part 16 and second end cap 6 betweenthem define an annular slot 22, in which the drainage layer can bereceived. The slot is tapered inwardly so that the distance between theflange and the surface of the end cap decreases progressively. The slotis therefore generally V-shaped when the end cap is viewed from oneside.

The bottom part of the drainage layer 12 is wrapped over the wall 18 ofthe trough 14 of the bottom end cap 6 and folded under its bottomsurface 20, in the annular slot 22 between the second end cap 6 and theflange part 16. The drainage layer 12 is fastened in the space 22 bymeans of a loop of elastic material 98, such as an elastic band orO-ring, which can be stretched to fit over the flange. The loop 98 issized so that it is pinched between the opposing surfaces of the slotand the drainage layer: the transverse dimension of the material of theloop is slightly greater than the distance between the drainage layerand the opposing surface of the slot when the loop the tension in theloop (resulting from stretching it to fit it over the flange) isrelaxed. This causes the loop and the drainage layer to be compressedslightly, so that the drainage layer is gripped in the annular slot. Thediameter of the flange is smaller than the diameter of the end cap sothat at least a part 30 of the drainage layer 12 folded over the bottomsurface 20 is exposed.

The bottom end cap 6 also has a central upstand portion 100. The centralupstand portion 100 extends from the bottom of the hollow space definedby the bottom end cap 6 towards the first end cap. The central upstandportion 100 has a generally cylindrical base 102, and a generallyconical part 104 extending from the base toward the first end cap 4. Thediameter of the cylindrical base 102 is the same as that of the stem 28.

Referring now to FIG. 1, an assembly according to the present inventionis shown which includes a housing 50 in which the filter element shownin FIG. 2 can be located when in use. However, as shown, an alternativeembodiment of a filter element which is substantially the same as thatshown in FIG. 2, is located within the housing.

The filter element shown in FIG. 1 is substantially the same as thatshown in FIG. 2, except that an extension 236 is provided on theexternal wall of the flow conduit 34 and extends away from the secondconduit opening 164, instead of the port 80 for connection to a gauge.The extension 236 has seating portion 238 having a generally circularcross-section, and a fin 240 having a generally planar configurationwhich extends between the seating portion 238 and the flow conduit 34.The seating portion 238 provides a generally flat surface 244 and a wall256 which extends around the periphery of the surface and away from thesecond conduit opening 164. A differential pressure measuring device(discussed in more details below) can be received by and containedwithin the area defined by the surface 244 and wall 256 of the seatingportion. The extension 236 is configured so that the axis of thecircular seating portion 238 is angled relative to the axis of thesecond conduit opening 164 by 5°. Therefore, as shown in FIG. 1, whenviewed in cross-section, the seating portion 238 appears slantedrelative to the filter element. A groove 242 that is capable ofreceiving an O-ring is formed around the periphery of the wall 256. Avent 246 that extends between the flow path 36 of the flow conduit 34and the surface 244 of the seating portion 238 is formed within theextension 236.

The housing comprises a head 52 and a body 54 which can be connected toone another by means of cooperating screw threads (as is wellestablished) at their interfaces 86, 88. The head and body are formedfrom a metallic material, especially aluminium or an alloy thereof. Theycan be formed by machining, or by techniques such as casting.

The housing body comprises a cylindrical wall 55, an end wall 57 at oneend of the internal wall, and an open end at the opposite end of thecylindrical wall. The housing body defines a space within which thefilter element is coaxially located when in use. Liquid drops whichdrain from the drainage layer are collected in a reservoir 60 in thehousing body. The housing includes a drain outlet 62, for example of thekind which is disclosed in European Patent No. EP-A-81826.

The filter element fits on to the housing body by means ofinter-engaging formations in the form of ribs and grooves. The top endcap 4 has first 90, second 91, third 154 and fourth 156 ribs (forexample as shown in FIGS. 4 and 5) around its perimeter that extend fromthe top end cap towards the bottom end cap, on the exterior of thefilter element. The four ribs are spaced apart about 90° around the topend cap. Further, the first 90 and second 91 ribs are spaced about 180°around the top end cap. The first 90 and second 91 ribs are received inthe housing body by means of correspondingly shaped and positioned first92, second 93, grooves provided in the interior of the housing body atthe open end.

The third and fourth ribs 154, 156 are identical in shape size andconfiguration. The leading edge of the third and fourth ribs 154, 156(which is directed into the gas stream) is rounded and the trailing edgeof the rib is tapered inwardly, towards (optionally to) a sharp edge orpoint. These ribs 154, 156 are approximately aerofoil-shaped when viewedin cross-section (perpendicular to the axis of the assembly). This shapegives minimal resistance to the flow of gas past the ribs. In contrastto the first 90 and second 91 ribs, there are no corresponding groovesfor the third 154 and fourth 156 ribs. Instead, the axial faces of thethird 154 and fourth 156 ribs match the profile of the housing body sothat they rest against the body when the filter element has been fittedwithin the housing body.

The first and second ribs 91, 90 are shaped differently to that of thirdand fourth ribs 154, 156. The leading edge of the first and second ribs90, 91 are rounded, and the trailing end is flared outward from therounded leading edge. The first and second ribs 90, 91 have anapproximately tapered “V” shape when viewed in cross-section(perpendicular to the axis of the assembly) with a flat top surfaceextending between the ends of the “V”.

As can be seen in FIG. 1, the second rib 91 is located under theinternal cylindrical wall 72 of the head 52 of the housing 50 (discussedin more detail below). Therefore, when the filter assembly is in use,the tapered sides of the second rib 91 aids the flow gas around theinternal cylindrical wall 72, and towards the outlet port 58. The firstrib 90 is located below the output port 58. Therefore, in order tominimise the direction of gas away from the output port, the width ofthe first rib 90 at its widest point is smaller than that of the secondrib 91. Further, due to the first rib 90 and its corresponding firstgroove 92 being narrower than the second rib 91 and its correspondingsecond groove 93, the filter element can only be inserted into the bodyin one orientation. This can ensure that when the housing is assembled,the inlet port 56 of the housing head 52 (described in more detailbelow) is aligned with the flow conduit 34 of the top end cap 4, ratherthan incorrectly aligned with the outlet port 58 of the housing head.This is especially true when the housing head and body are configured sothat they can only fit together in one orientation. This can be achievedby providing a single start screw thread at the interfaces 86, 88 of thehousing head and body.

The filter element is assembled in the housing body by locating thefirst and second ribs 90, 91 with the first and second grooves 92, 93 ofthe housing body 54, and then sliding the ribs into the grooves untilthey sit on the bottom of the grooves. Once the ribs have been fullyreceived by the grooves, the filter element is securely suspended withinthe housing body. Therefore, as will be appreciated, the axial positionof the filter element within the housing body can be controlled by theshape and size of the ribs and grooves. Further, rotation of the filterelement relative to the housing body is inhibited by the interlocking ofthe ribs with the grooves.

Once the filter element is appropriately assembled in the housing body,an annular space 64 is defined between the filter element and thehousing. The filter element can be removed from the housing body 54, bypulling the filter element away from the housing body along its axis.

The housing body 54 has two a plurality of fins 96 extending along thecylindrical wall 55, parallel to the axis of the housing body. Thenumber of fins 96 provided on the cylindrical wall 55 of the housingbody 54 can depend on the size of the housing body. For example, ingeneral, the larger the housing body, the greater the number of fins 96provided. Typically, the minimum number of fins 96 provided will be two.In the embodiment shown, the housing body has six fins 96, however, onlytwo are shown. The fins extend from the end wall 57 of the housing bodytowards the open end, and are spaced uniformly around the cylindricalwall 55 of the housing body 54. When the filter element is assembled inthe housing body so that the ribs have been fully received within thegrooves, the part of the drainage layer 12 which extends over the bottomend cap 6 is compressed between the bottom end cap and the edge of thefins 96. Accordingly, the transverse position of the filter elementwithin the housing body can be controlled by the shape and size of thefins. The cross-section of the edge of the fins which contacts thedrainage layer, taken perpendicular to the length of the ridge, is arounded convex shape. As a result of the local compression of thedrainage layer between the fins and the second end cap, liquid collectedin the drainage layer is encouraged to drain from it, along each fin.

The housing head includes an inlet port 56 which communicates with theflow conduit 34 on the top end cap 4 through a transversely extendingprimary chamber 68 within the housing head. The primary chamber 68 isdefined by an internal cylindrical wall 72 extending transversely withinthe housing head, and an internal end wall 70 opposite the inlet port56. The internal cylindrical side wall 72 and end wall 70 are integralto the housing head. A first circular aperture 74, coaxial with thehousing head, is defined within the part of side wall of the inletconduit that is proximal to the filter element when assembled. A recess248, that extends away from the filter element when the assembly isassembled, and that is coaxial with the housing head, is formed withinthe primary chamber 68. Further, a vent 250 that extends between theprimary chamber 68 and the area surrounding it is formed in the end wall70.

The housing head 52 is secured to the body 54 (once the filter elementhas been located in the housing body) by locating the flow conduit 34 ofthe top end cap 4 in the primary chamber 68 of the housing head throughthe circular aperture 74. The flow conduit 34 has an O-ring 66 on itsexternal surface which is received by the aperture 74, in which it iscompressed to form a fluid tight seal. The seating portion 238 also hasan O-ring 252 which is compressed between the groove 242 of the seatingportion and the side wall of the recess 248 within primary chamber 68,to form a fluid tight seal, thereby defining an auxiliary chamber 254within the primary chamber.

The housing head 52 and body 54 are secured by rotating one relative tothe other so that their cooperating screw threads at their interfaces86, 88, are tightened to interlock with each other. Once the head isproperly secured to the body 54 the vent 250 in the end wall 70 of thehousing head 52 extends between the auxiliary chamber 254 and the areawithin the housing head surrounding the primary chamber 68.

A device 258 for measuring differential pressure is contained within thearea defined by the surface 244 and the wall 256 of the seating portion238. Suitable differential pressure measuring devices are known inexisting products, for example in products sold by Domnick HunterLimited under the trade mark OIL-X. The device 258 can be secured to theseating portion 238 by frictional forces between it and the wall 256 ofthe seating portion 238; i.e. the device is a secured to the seatingportion 238 by a “press-fit” connection. However, other techniques canbe used to secure the pressure measuring device, for example a threadedengagement or an adhesive. An O-ring 260 is provided on the pressuremeasuring device 258 which is compressed between the device and the wall256 to provide a fluid tight seal between the vent 246 within theextension 236 and the auxiliary chamber 254. As the vent 246 within theextension 236 is in fluid communication with gas upstream of the filterelement and the auxiliary chamber 254 is in fluid communication with gasdownstream of the filter element via the vent 250 in the end wall 70,the pressure measuring device is capable of measuring the pressure dropacross the filter element.

The filter assembly can be disassembled by rotating the housing body 54relative to the head 52 so that their cooperating screw threads areloosened. Any rotational force that is imparted on the top end cap 4 ofthe filter element by frictional forces between the O-rings 66, 252 ofthe end cap 4 and the housing head is negated by the opposite rotationaldrive that is provided by the first and second ribs 90, 91 actingagainst the first and second grooves 92, 93 in the housing body in whichthe ribs are received. Therefore, as the housing body 54 is rotatedrelative to the housing head 52, the filter element will tend to residein the housing rather than be drawn away from the body with the head,and hence when the housing head 52 is removed from the housing body 54,the filter element will remain located within the body, rather thanbeing removed from the body with the head.

The housing head includes an outlet port 58 through which gas which haspassed through the wall 2 of the filter element can be supplied to adownstream application. The outlet port communicates with the annularspace 64 between the wall of the filter element and the internal wall ofthe housing.

In use, a gas that is to be filtered enters the filter assembly throughthe inlet port 56 in the housing head and is directed to the hollowspace 8 in the filter element by means of the primary chamber 68 in thehousing head and the flow paths, 36 and 109 in the flow conduit of thefilter element, and the inlet tube, respectively. A helical flow isimparted in the gas stream entering the hollow space 8 by the helicallyextending ridge 110, as the gas stream passes through the inlet tube.The supply of gas entering the hollow space 8 is graded due to thegradual increase in the proportion of the inlet tube that is opentowards its downstream end.

From the hollow space 8, the gas flows generally radially outwardlythrough the filter medium of the wall 2. Any liquid in the gas streamwill be coalesced by the filtration layer 10 and any coalesced liquidwill be carried to the drainage layer 12 by the flow of gas, where theliquid will be retained. The liquid will tend to drain to the bottom ofthe drainage layer 12, where it can tend to accumulate in the part ofthe drainage layer 12 folded under the bottom surface 12, therebyforming a wet band. When that part of the drainage layer 12 becomessufficiently saturated, the liquid will begin to drain from any exposedparts of that part of the drainage layer, generally in the form ofdrops. The compression of the drainage layer 12 by the fins 96 will tendto encourage the drainage of liquid from the drainage layer along thefins.

Filtered gas exiting the filter element enters the annular space 64between the filter element and the housing. Filtered gas is thendischarged from the filter assembly through the outlet port 58 in thehousing head 52, which is in fluid communication with the annular space64. Gas flowing from the annular space 64 to the outlet port 58 isdirected around the internal cylindrical wall 72 of the housing head 52,and toward the outlet port, by the tapered rib 91 that is locateddirectly underneath the cylindrical side wall.

1. A filter assembly for removing material that is entrained in a gasstream, which comprises: a filter element comprising a wall of afiltration medium which defines a hollow space, for a gas stream to flowfrom the space through the wall to be filtered, the wall including afiltration layer, and a drainage layer located outside the filtrationlayer in which liquid separated from the gas stream can collect, andfirst and second end caps at opposite ends of the wall, the first endcap including an inlet for a gas stream to be supplied to the spacewithin the wall, and a housing in which the filter element can bereceived when in use, the housing having an inlet port for a gas streamto flow into the housing which communicates with the inlet in the firstend cap, and an outlet port through which gas which has passed throughthe filtration medium can leave the housing, in which the drainage layerextends over at least a part of the external surface of the second endcap between the second end cap and the housing and in which the part ofthe drainage layer between the second end cap and the housing is locallycompressed by at least one longitudinally extending fin.
 2. A filterassembly as claimed in claim 1, in which the angle between the fin andthe axis of the housing is not more than about 45°.
 3. A filter assemblyas claimed in claim 1, in which the fin follows a helical path relativeto the axis of the housing.
 4. A filter assembly as claimed in claim 1,in which the fin extends approximately parallel to the axis of thehousing.
 5. A filter assembly as claimed in claim 1, in which the edgeof the fin which acts against the filtration medium directly orindirectly is generally rounded.
 6. A filter assembly as claimed inclaim 1, in which the fin is tapered towards its end.
 7. A filterassembly as claimed in claim 1, in which the drainage layer iscompressed between the second end cap and the housing wall by means ofat least two longitudinally extending fins.
 8. A filter assembly asclaimed in claim 7, in which the fins are spaced apart around the filterelement approximately uniformly.
 9. A filter assembly as claimed inclaim 1, in which the fin is provided on the internal wall of thehousing.
 10. A filter assembly as claimed in claim 9, in which the finis formed integrally with the wall of the housing.
 11. A filter assemblyas claimed in claim 1, in which the length of the fin, measured from thesecond end of the filter element towards the first end thereof, is atleast about 1.0 cm.
 12. A filter assembly as claimed in claim 1, inwhich the ratio of the length of the fin, measured from the second endof the filter element towards the first end thereof to the length of thefilter element is at least about 0.02.
 13. A filter assembly as claimedin claim 1, in which the length of the fin, measured from the second endof the filter element towards the first end thereof, is not more thanabout 20 cm.
 14. A filter assembly as claimed in claim 1, in which theratio of the length of the fin measured from the second end of thefilter element towards the first end thereof to the length of the filterelement is at most about 0.8.
 15. A filter assembly as claimed in claim1, in which the second end cap provides a trough in which the filtrationlayer is retained, and in which the drainage layer is wrapped around theoutside of the end cap.
 16. A filter assembly as claimed in claim 15, inwhich the transverse dimension of the element where the drainage layeris wrapped around the end cap is greater than the transverse dimensionof the element between the first and second end caps.
 17. A filterassembly as claimed in claim 1, in which the inside of the housing isapproximately circular in cross-section, and in which the anglesubtended at the centre of the housing by the fin is not more than about10°.
 18. A filter assembly as claimed in claim 1, which includes a clampfor fastening the drainage layer to the surface of the second end capwhich faces away from the first end cap.
 19. A filter assembly asclaimed in claim 18, in which the clamp has a flange and in which thedrainage layer is clamped between the flange and the second end cap. 20.A filter assembly as claimed in claim 1, in which the second end capincludes a flange which defines an annular slot in which the drainagelayer can be inserted to retain it in place over the external surface ofthe end cap.
 21. A filter assembly as claimed in claim 20, whichincludes a ring member which can be fitted into the annular slot overthe drainage layer to retain the drainage layer in the slot.
 22. Atubular filter element for collecting material that is entrained in agas stream, which has a wall of a filtration medium which defines ahollow space within it so that a gas stream can flow through the wall tobe filtered, the wall including a filtration layer and a drainage layerlocated outside the filtration layer in which liquid separated from thegas stream can collect, the element including first and second end capsat opposite ends of the wall, the first end cap including an inlet for agas stream to be supplied to the space within the wall, the drainagelayer extending over at least a part of the external surface of thesecond end cap, and in which the element includes a fin assembly with atleast two fins extending along the filter element, externally of thedrainage layer, to provide localised compression of the drainage layerwhen the element is located within a filter housing and the fins contactthe internal wall of the housing.
 23. A filter element, including a ringof filter media with first and second ends circumscribing a centrallongitudinal axis and defining a central cavity, the filter mediaincluding a filtration layer and a drainage layer surrounding thefiltration layer, the first end cap including an opening to the centralcavity, the drainage layer extending over at least a part of an externalend surface of the second end cap, and in which the element includes afin assembly with at least two fins extending along the filter element,externally of the drainage layer, to provide localized compression ofthe drainage layer when the element is located within a filter housingand the fins contact an internal wall of the housing.